Although the present invention is applicable to any micromechanical component, the present invention is explained on the basis of actuators for micromirrors.
Micromirrors are used for deflecting optical beams, for example in bar code scanners, projection systems or for switching optical data links. There are two basic modes of operation for micromirror deflection. In the resonant mode of operation, a natural frequency of the micromirror is induced. This permits very high mirror inclination angles at low energy injection levels.
The disadvantage is that the oscillation frequency is always set to the range around the micromirror resonance frequency, and the oscillation therefore always describes a sinusoidal function. In projection applications, this causes a poor distribution of the image intensity, since the beam scans the center of the image very quickly, while it scans the edge of the image very slowly.
For two-dimensional scanners, a Lissajous figure is also typically employed. This means that a great deal of computing power is needed during image preparation for controlling the projection laser. The image resolution does not achieve the levels that would result from the horizontal oscillation frequency of the micromirror in line-by-line projection. In addition, an image which is “chaotically” structured in this manner is unpleasant for the viewer, since it appears to disintegrate when the viewer moves his/her head quickly.
For line-by-line projection of video images, in which these problems do not occur, a micromirror which may be statically or quasi-statically excited on at least one axis is required. Since the electrostatic force may have only an attracting effect, but not a repelling effect, electrode structures on one plane, which are frequently used for resonant micromirrors, are unusable. In contrast, the electrodes on different planes are suitable for implementing a quasi-static electrostatic drive. Several approaches exist for this purpose.
PCT Application No. WO 2008-071172 describes a micromirror actuator which has, in an inner region, at least one movable micromirror element and multiple finger-shaped or comb-shaped electrodes for activating the micromirror, which are offset against each other in height, the actuator being formed from a layer structure having at least three main layers, at least sections of which are electrically insulated from each other by intermediate layers. This type of system of comb electrodes on parallel planes located on top of each other is also known as an OOP comb system (out-of-plane combs).
German Patent Application No. DE 10 2008 003 344.8 describes angled, vertical combs as a micromirror actuator, the combs being structured on one plane and subsequently predeflected from the plane.
The torque generally increases as the application of force moves farther away from a rotation axis. In electrostatic actuators which execute a tilting motion of a structure from a plane, for example against a spring, the vertical comb structures or finger structures should therefore be located as far as possible from the rotation axis. However, this has limits, since the achievable deflection depends on the distance between the electrode pairs (rotor and stator) and the rotation axis at a given depth of OOP comb electrodes. The greater the distance between the electrodes and the rotation axis, the sooner does the rotor disengage from the stator. Since no further energy gain is achievable from this point on, this point corresponds to the maximum deflection. In other words, the maximum deflection is lower in OOP electrode structures located at a greater distance from the rotation axis than it is in OOP electrode structures located closer to the rotation axis.
FIGS. 8a and 8b are schematic cross-sectional views of two comb electrode teeth for illustrating a conventional micromechanical comb drive structure.
In FIGS. 8a and 8b, reference numeral E1 designates a first plane on which is provided a comb electrode having comb electrode teeth Fd which may rotate or swivel around an anchoring point V, the comb electrode being located parallel to a second comb electrode, which has stationary comb electrode teeth Ff and which is provided on a second underlying plane E2. Planes E1, E2 are located parallel to each other.
Upon application of a voltage U, comb electrode teeth Fd are inserted into stationary comb electrode teeth Ff. This causes an energy reduction, since the overlapping area of the two electrodes Ff, Fd is enlarged during insertion. A corresponding torque therefore arises at rotatable comb electrode Fd. The torque curve is stable up to angle α illustrated in FIG. 8b, at which rotatable comb electrode Fd is fully introduced into stationary comb electrode Ff. The area overlap no longer increases uniformly beyond angle α. On the bottom, rotatable comb electrode Fd reemerges from the overlap at angles greater than α. This is apparent in the torque curve by a strong drop in torque.
This discussion clearly shows that angle of inclination a has a limit for OOP comb electrodes which is defined by the length and height of the comb electrode fingers. High comb electrodes permit a large angle of inclination, but are more difficult to manufacture than low comb electrode fingers, which are manufactured, for example, in a trench-etching process. The use of long comb electrode fingers may greatly increase the torque, while possible deflection angle α decrease along with the finger length.
FIG. 9 is a schematic cross-sectional view of a conventional micromechanical comb drive structure for a rotary drive of a micromirror.
In FIG. 9, reference numeral VS designates a mount, for example a stationary region of a substrate. A comb electrode rotary drive A is connected via a torsion spring f0, a micromirror S, which is rotatable about a rotation axis DA0, along which torsion spring f0 runs and provides a corresponding restoring torque, being attached to the other side of the comb electrode rotary drive. Comb electrode rotary drive A includes a first rotatable comb electrode Da having rotatable comb electrode teeth Fd and a diametrically opposed, second rotatable comb electrode Db having rotatable comb electrode teeth Fd. Rotatable comb electrodes Da, Db interact with corresponding stationary comb electrodes Fa, Fb, which are also attached to mount VS and have stationary comb electrode teeth Ff. With reference to FIG. 8, rotatable comb electrodes Da, Db are located on plane E1, and stationary comb electrodes Fa, Fb are located on plane E2.
By applying a voltage difference U between first rotatable comb electrode Da and first stationary comb electrode Fa or at second rotatable comb electrode Db and second stationary comb electrode Fb, a rotation around rotation axis DA0 is possible, by an angle of up to +α or −α in each case (see FIG. 8).